Ole Lajord Munk

Brain metabolism of 13N‐ammonia during acute hepatic encephalopathy in cirrhosis measured by positron emission tomography

Animal studies and results from 13N‐ammonia positron emission tomography (PET) in patients with cirrhosis and minimal hepatic encephalopathy suggest that a disturbed brain ammonia metabolism plays a pivotal role in the pathogenesis of hepatic encephalopathy (HE). We studied brain ammonia kinetics in 8 patients with cirrhosis with an acute episode of clinically overt HE (I‐IV), 7 patients with cirrhosis without HE, and 5 healthy subjects, using contemporary dynamic 13N‐ammonia PET. Time courses were obtained of 13N‐concentrations in cerebral cortex, basal ganglia, and cerebellum (PET‐scans) as well as arterial 13N‐ammonia, 13N‐urea, and 13N‐glutamine concentrations (blood samples) after 13N‐ammonia injection. Regional 13N‐ammonia kinetics was calculated by non‐linear fitting of a physiological model of brain ammonia metabolism to the data. Mean permeability–surface area product of 13N‐ammonia transfer across blood–brain barrier in cortex, PSBBB, was 0.21 mL blood/min/mL tissue in patients with HE, 0.31 in patients without HE, and 0.34 in healthy controls; similar differences were seen in basal ganglia and cerebellum. Metabolic trapping of blood 13N‐ammonia in the brain showed neither regional, nor patient group differences. Mean net metabolic flux of ammonia from blood into intracellular glutamine in the cortex was 13.4 μmol/min/L tissue in patients with cirrhosis with HE, 7.4 in patients without HE, and 2.6 in healthy controls, significantly correlated to blood ammonia. In conclusion, increased cerebral trapping of ammonia in patients with cirrhosis with acute HE was primarily attributable to increased blood ammonia and to a minor extent to changed ammonia kinetics in the brain. (HEPATOLOGY 2005;43:42–50.)

Dynamic FDG-PET is useful for detection of cholangiocarcinoma in patients with PSC listed for liver transplantation.

Five to 15% of patients with primary sclerosing cholangitis (PSC) develop cholangiocarcinoma (CC) with a median survival of 5 to 7 months, an outcome not significantly improved by liver transplantation. However, if CC is found incidentally during the procedure or in the explanted liver, 5‐year survival rates of 35% are reported. A noninvasive method to detect CC small enough to allow for intended curative surgery is needed. Unfortunately, computed tomography (CT) and ultrasonography (US) have poor sensitivity for detection of CC in PSC; however, positron emission tomography (PET) using 2‐[18F]fluoro‐2‐deoxy‐D‐glucose (FDG) differentiates well between CC and nonmalignant tissue. We examined whether PET findings are valid using a blinded study design comparing pretransplantation FDG‐PET results with histology of explanted livers. Dynamic FDG‐PET was performed in 24 consecutive patients with PSC within 2 weeks after listing for liver transplantation and with no evidence of malignancy on CT, magnetic resonance imaging, or ultrasonography. The PET Center staff was blinded to clinical findings, and surgeons and pathologists were blinded to the PET results. Three patients had CC that was correctly identified by PET. PET was negative in 1 patient with high‐grade hilar duct dysplasia. In 20 patients without malignancies, PET was false positive in 1 patient with epitheloid granulomas in the liver. In conclusion, dynamic FDG‐PET appears superior to conventional imaging techniques for both detection and exclusion of CC in advanced PSC. FDG‐PET may be useful for screening for CC in the pretransplant evaluation of patients with PSC. (HEPATOLOGY 2006;44:1572–1580.)

Imaging acetylcholinesterase density in peripheral organs in Parkinson's disease with 11C-donepezil PET

Parkinsons disease is associated with early parasympathetic dysfunction leading to constipation and gastroparesis. It has been suggested that pathological α-synuclein aggregations originate in the gut and ascend to the brainstem via the vagus. Our understanding of the pathogenesis and time course of parasympathetic denervation in Parkinsons disease is limited and would benefit from a validated imaging technique to visualize the integrity of parasympathetic function. The positron emission tomography tracer 5-[(11)C]-methoxy-donepezil was recently validated for imaging acetylcholinesterase density in the brain and peripheral organs. Donepezil is a high-affinity ligand for acetylcholinesterase-the enzyme that catabolizes acetylcholine in cholinergic synapses. Acetylcholinesterase histology has been used for many years for visualizing cholinergic neurons. Using 5-[(11)C]-methoxy-donepezil positron emission tomography, we studied 12 patients with early-to-moderate Parkinsons disease (three female; age 64 ± 9 years) and 12 age-matched control subjects (three female; age 62 ± 8 years). We collected clinical information about motor severity, constipation, gastroparesis, and other parameters. Heart rate variability measurements and gastric emptying scintigraphies were performed in all subjects to obtain objective measures of parasympathetic function. We detected significantly decreased (11)C-donepezil binding in the small intestine (-35%; P = 0.003) and pancreas (-22%; P = 0.001) of the patients. No correlations were found between the (11)C-donepezil signal and disease duration, severity of constipation, gastric emptying time, and heart rate variability. In Parkinsons disease, the dorsal motor nucleus of the vagus undergoes severe degeneration and pathological α-synuclein aggregations are also seen in nerve fibres innervating the gastro-intestinal tract. In contrast, the enteric nervous system displays little or no loss of cholinergic neurons. Decreases in (11)C-donepezil binding may, therefore, represent a marker of parasympathetic denervation of internal organs, but further validation studies are needed.

Identifying hypoxia in human tumors: A correlation study between 18F-FMISO PET and the Eppendorf oxygen-sensitive electrode

Abstract Introduction. Polarographic oxygen-sensitive electrodes have demonstrated prognostic significance of hypoxia. However, its routine application is limited. 18F-FMISO PET scans are a noninvasive approach, able to measure spatial and temporal changes in hypoxia. The aim of this study was to examine the association between measures of hypoxia defined by functional imaging and Eppendorf pO2 electrodes. Materials and methods. A total of 18 patients were included, nine squamous cell carcinoma of the head and neck and nine soft tissue tumors. The tumor volume was defined by CT, MRI, 18FDG-PET or by clinical examination. The oxygenation status of the tumors was assessed using 18F-FMISO PET imaging followed by Eppendorf pO2 electrode measurements. Data were compared in a ‘virtual voxel’, resulting in individual histograms from each tumor. Results. The percentages of pO2 ≤ 5 mmHg ranged from 9 to 94% (median 43%) for all 18 tumors. For 18F-FMISO PET the T/M ratio ranged from 0.70 to 2.38 (median 1.13). Analyzing the virtual voxel histograms tumors could be categorized in three groups: Well oxygenated tumors with no hypoxia and concordance between the 18F-FMISO data and the Eppendorf measurements, hypoxic tumors likewise with concordance between the two assays and inconclusive tumors with no concordance between the assays. Conclusion. This study analyzed the relationship between 18F-FMISO PET and Eppendorf pO2 electrode measurements by use of a virtual voxel model. There was a spectrum of hypoxia among tumors that can be detected by both assays. However no correlation was observed, and in general tumors were more hypoxic based on Eppendorf pO2 measurements as compared to 18F-FMISO PET.

Hepatic encephalopathy is associated with decreased cerebral oxygen metabolism and blood flow, not increased ammonia uptake

Studies have shown decreased cerebral oxygen metabolism (CMRO2) and blood flow (CBF) in patients with cirrhosis with hepatic encephalopathy (HE). It remains unclear, however, whether these disturbances are associated with HE or with cirrhosis itself and how they may relate to arterial blood ammonia concentration and cerebral metabolic rate of blood ammonia (CMRA). We addressed these questions in a paired study design by investigating patients with cirrhosis during and after recovery from an acute episode of HE type C. CMRO2, CBF, and CMRA were measured by dynamic positron emission tomography (PET)/computed tomography (CT). Ten patients with cirrhosis were studied during an acute episode of HE; nine were reexamined after recovery. Nine patients with cirrhosis with no history of HE served as controls. Mean CMRO2 increased from 0.73 μmol oxygen/mL brain tissue/min during HE to 0.91 μmol oxygen/mL brain tissue/min after recovery (paired t test; P < 0.05). Mean CBF increased from 0.28 mL blood/mL brain tissue/min during HE to 0.38 mL blood/mL brain tissue/min after recovery (P < 0.05). After recovery from HE, CMRO2 and CBF were not significantly different from values in the control patients. Arterial blood ammonia concentration decreased 20% after recovery (P < 0.05) and CMRA was unchanged (P > 0.30); both values were higher than in the control patients (both P < 0.05). Conclusion: The low values of CMRO2 and CBF observed during HE increased after recovery from HE and were thus associated with HE rather than the liver disease as such. The changes in CMRO2 and CBF could not be linked to blood ammonia concentration or CMRA. (HEPATOLOGY 2013)

Assessing hypoxia in animal tumor models based on pharmocokinetic analysis of dynamic FAZA PET

Abstract Positron emission tomography (PET) allows non-invasive detection and mapping of tumor hypoxia. However, slow tracer kinetics and low resolution, results in limited tumor-to-normal tissue contrast and the risk of missing areas where hypoxic cells are intermixed with necrosis. The shape of tumor time activity curves (TACs), as deduced from dynamic scans, may allow further separation of tumors/tumor sub-volumes that are inseparable based on static scans. This study was designed to define the added value of dynamic scans. Material and methods. Three squamous cell carcinoma tumor models were grown in mice. Mice were injected with the 18F-labeled PET hypoxia-tracer fluoroazomycin arabinoside (FAZA) and the immunologically-detectable hypoxia-marker pimonidazole, and PET scanned dynamically for three to six hours. Subsequently, microregional tracer retention (autoradiography) and the distribution of pimonidazole-retaining cells (immunohistology) and necrosis were analyzed in tumor tissue sections. Dynamic PET data were analysed based on a two-compartment model with irreversible tracer binding generating estimates of the putative hypoxia surrogate markers k3 (tracer trapping rate constant) and Ki (influx rate constant from plasma into irreversible bound tracer). Results/Discussion. High tumor-to-reference tissue ratios and a strong linear correlation (R∼0.7 to 0.95) between density of hypoxic cells and FAZA concentration was observed three hours after tracer administration, suggesting that late time PET images provides an accurate measure of hypoxia against which kinetic model estimates can be validated. Tumor TACs varied widely (ranging from distinctly wash-out to accumulative type) among tumor types although pimonidazole-stainings revealed extensive hypoxia in all models. Kinetic analysis of tumor sub-volumes showed that k3 correlated poorly with late time FAZA retention regionally in two of the three tumor models. The influx rate constant Ki displayed far less variability and correlated strongly with late time FAZA retention (hypoxia) in two of three tumor models, whereas a non-consistent relationship was observed in the last tumor model. Our study demonstrates the potential usefulness of dynamic PET, but also that a simple two-compartment model may be inappropriate in some tumor models.

Hepatic uptake and metabolism of galactose can be quantified in vivo by 2-[18F]fluoro-2-deoxygalactose positron emission tomography

Metabolism of galactose is a specialized liver function. The purpose of this PET study was to use the galactose analog 2-[(18)F]fluoro-2-deoxygalactose (FDGal) to investigate hepatic uptake and metabolism of galactose in vivo. FDGal kinetics was studied in 10 anesthetized pigs at blood concentrations of nonradioactive galactose yielding approximately first-order kinetics (tracer only; n = 4), intermediate kinetics (0.5-0.6 mmol galactose/l blood; n = 2), and near-saturation kinetics (>3 mmol galactose/l blood; n = 4). All animals underwent liver C15O PET (blood volume) and FDGal PET (galactose kinetics) with arterial and portal venous blood sampling. Flow rates in the hepatic artery and the portal vein were measured by ultrasound transit-time flowmeters. The hepatic uptake and net metabolic clearance of FDGal were quantified by nonlinear and linear regression analyses. The initial extraction fraction of FDGal from blood-to-hepatocyte was unity in all pigs. Hepatic net metabolic clearance of FDGal, K(FDGal), was 332-481 ml blood.min(-1).l(-1) tissue in experiments with approximately first-order kinetics and 15.2-21.8 ml blood.min(-1).l(-1) tissue in experiments with near-saturation kinetics. Maximal hepatic removal rates of galactose were on average 600 micromol.min(-1).l(-1) tissue (range 412-702), which was in agreement with other studies. There was no significant difference between K(FDGal) calculated with use of the dual tracer input (Kdual(FDGal)) or the single arterial input (Karterial(FDGal)). In conclusion, hepatic galactose kinetics can be quantified with the galactose analog FDGal. At near-saturated kinetics, the maximal hepatic removal rate of galactose can be calculated from the net metabolic clearance of FDGal and the blood concentration of galactose.

Dynamic 2-[18F]fluoro-2-deoxy-D-glucose positron emission tomography of liver tumours without blood sampling

Abstract.Positron emission tomography (PET) using 2-[18F]fluoro-2-deoxy-d-glucose (FDG) is a useful diagnostic tool for the detection of tumours. Using dynamic FDG PET, net metabolic clearance of FDG, K, can be calculated by Gjedde-Patlak analysis of the time course of the radioactivity concentrations in tissue and arterial blood. We examined whether time-activity curves (TACs) based on arterial blood sampling could be replaced by TACs obtained from the descending aorta in dynamic PET scans of patients with liver tumours. The study was performed in two parts, using data from dynamic liver scans with arterial blood sampling in human subjects: First, data from four patients with no liver tumours and five patients with liver tumours were used as a training group. Volumes of interest were defined in the descending aorta (aorta VOIs) by four different methods. K values were calculated based on the corresponding TACs and compared with those based on TACs of the arterial blood sample radioactivity concentrations. The aorta VOI which gave K values that were in best agreement with the K values based on the arterial blood sample measurements was called the AORTA-VOI. Use of the AORTA-VOI was subsequently tested in a test group of 19 tumour patients by comparing the K values from the AORTA-VOI with the K values based on the arterial blood sample measurements. The AORTA-VOI consisted of the sum of small regions of interest (ROIs) drawn in the centre of the aorta (approximately six pixels of 2.4×2.4 mm per transaxial slice of 3.1 mm thickness) in as many transaxial slices as possible (30–40 slices). There were no statistically significant differences between the two sets of K values. The ratio of K values in tumour tissue to K values in reference tissue was 2.1–9.7:1 (mean, 5.4:1) based on the AORTA TACs, and 2.1–8.4:1 (mean, 4.6:1) based on blood sample TACs (P>0.3). We conclude that arterial blood sampling can be replaced by the present AORTA-VOI in the calculation of the net metabolic clearance of FDG in dynamic PET studies of liver tumours in human subjects.

[11C]-Labeled Metformin Distribution in the Liver and Small Intestine Using Dynamic Positron Emission Tomography in Mice Demonstrates Tissue-Specific Transporter Dependency.

Metformin is the most commonly prescribed oral antidiabetic drug, with well-documented beneficial preventive effects on diabetic complications. Despite being in clinical use for almost 60 years, the underlying mechanisms for metformin action remain elusive. Organic cation transporters (OCT), including multidrug and toxin extrusion proteins (MATE), are essential for transport of metformin across membranes, but tissue-specific activity of these transporters in vivo is incompletely understood. Here, we use dynamic positron emission tomography with [11C]-labeled metformin ([11C]-metformin) in mice to investigate the role of OCT and MATE in a well-established target tissue, the liver, and a putative target of metformin, the small intestine. Ablation of OCT1 and OCT2 significantly reduced the distribution of metformin in the liver and small intestine. In contrast, inhibition of MATE1 with pyrimethamine caused accumulation of metformin in the liver but did not affect distribution in the small intestine. The demonstration of OCT-mediated transport into the small intestine provides evidence of direct effects of metformin in this tissue. OCT and MATE have important but separate roles in uptake and elimination of metformin in the liver, but this is not due to changes in biliary secretion. [11C]-Metformin holds great potential as a tool to determine the pharmacokinetic properties of metformin in clinical studies.

In Vivo Imaging of Human Acetylcholinesterase Density in Peripheral Organs Using 11C-Donepezil: Dosimetry, Biodistribution, and Kinetic Analyses

Brain cholinergic function has been previously studied with PET but little effort has been devoted to imaging peripheral organs. Many disorders, including diabetes and Parkinson disease, are associated with autonomic dysfunction including parasympathetic denervation. Nonneuronal cholinergic signaling is also involved in immune responses to infections and in cancer pathogenesis. 5-11C-methoxy-donepezil, a noncompetitive acetylcholinesterase ligand, was previously validated for imaging cerebral levels of acetylcholinesterase. In the present study, we explored the utility of 11C-donepezil for imaging acetylcholinesterase densities in peripheral organs, including the salivary glands, heart, stomach, intestine, pancreas, liver, and spleen. Methods: With autoradiography, we determined binding affinities and levels of nonspecific 11C-donepezil binding to porcine tissues. Radiation dosimetry was estimated by whole-body PET of a single human volunteer. Biodistribution and kinetic analyses of 11C-donepezil time–activity curves were assessed with dynamic PET scans of 6 healthy human volunteers. A single pig with bacterial abscesses was PET-scanned to explore 11C-donepezil uptake in infections. Results: Autoradiography showed high 11C-donepezil binding (dissociation constant, 6–39 nM) in pig peripheral organs with low nonspecific signal. Radiation dosimetry was favorable (effective dose, 5.2 μSv/MBq). Peripheral metabolization of 11C-donepezil was low (>90% unchanged ligand at 60 min). Slow washout kinetics were seen in the salivary glands, heart, intestines, pancreas, and prostate. A linear correlation was seen between 11C-donepezil volumes of distribution and standardized uptake values, suggesting that arterial blood sampling may not be necessary for modeling uptake kinetics in future 11C-donepezil PET studies. High standardized uptake values and slow washout kinetics were seen in bacterial abscesses. Conclusion: 11C-donepezil PET is suitable for imaging acetylcholinesterase densities in peripheral organs. Its uptake may potentially be quantitated with static whole-body PET scans not requiring arterial blood sampling. We also demonstrated high 11C-donepezil binding in bacterial abscesses. We propose that 11C-donepezil PET imaging may be able to quantify the parasympathetic innervation of organs but also detect nonneuronal cholinergic activity in infections.